a. Field of the Invention
This invention relates to measuring the thickness of a sheet of metal alloy during its production. More particularly, the invention relates to a thickness measurement system which directs X-radiation toward the sheet, detects both the intensity of radiation transmitted therethrough and the intensity of radiation backscattered therefrom, and produces electrical signals expressive of said intensities that, when appropriately processed, yield a measurement of sheet thickness that accounts for small changes in composition of the sheet.
b. Background Art
Systems for determining the thickness of metal sheet via non-contacting radiation gauging techniques are well known. Such systems typically employ some form of the Bouguer Law: EQU I=I.sub.o e.sup.-mdt ( 1)
where "I.sub.o " is the intensity of radiation incident upon the sheet, "I" is the intensity of radiation transmitted through the sheet, "m" is the mass attenuation coefficient of the sheet, and "d" and "t" are the density and thickness, respectively, of the sheet. Equation 1 may be expressed as follows: EQU -1n (I/I.sub.o)=m(dt) (2)
This relation approximately holds where the composition of the sheet is uniform and relatively constant. To make the relationship more precise, "m" is typically expressed as a polynomial function of the logarithm of transmittance (I/I.sub.o). This is necessitated by the fact that the value of "m" will change with thickness. Accordingly, the system is typically calibrated by measuring the transmittance for several samples of known composition but varying thickness, and using the measurements to derive values for the polynomial function. If the sheet to be measured has a composition that is known and uniform, the value of "d" is known and the transmittance of the sheet can be measured to produce, by appropriate manipulation of Equation 2, a measurement of the sheet thickness.
The measurement problem is more complicated when the sheet is an alloy and the alloy composition varies. Although the variation is typically very small, a small variation in composition can have a pronouncedly adverse effect on the thickness measurement.
U.S. Pat. No. 4,047,029 Allport discloses a self-compensating X-ray thickness gauge that uses a combination of transmission and scattering measurements to compute thickness. Thickness is computed via an intermediate step of computing a mass attenuation coefficient that is corrected for deviations from the composition of a standard reference alloy. The calibration process requires two sets of samples (standards), and the described measurement system embodies an assumption that changes in density resulting from deviations in composition can be ignored.